misc/adios2: fix build with GCC-based architectures

Needs C++11 compiler to build:

target_compile_features The compiler feature "cxx_auto_type" is not known
to CXX compiler

"GNU"

version 4.2.1.
It also tries to link to libepoll-shim.so (installed by libfabric), so link to it and and devel/libepoll-shim to LIB_DEPENDS.

PR: 239635
Approved by: yuri (maintainer), linimon (mentor)
Differential Revision: https://reviews.freebsd.org/D21400